I'm very curious about IMS - a mentor described it as a graph database of sorts back in its day. I was told it was a great system (at least for its time). Does anyone have a perspective?
One of my first projects was to interface with an IMS system using Python of all things, there was only one guy that really knew the system and I had to sit with them for hours to even begin wrapping my head around how a hierarchical DBMS worked, how space is managed, etc. I remember the first comment he made was that the database was created for use as an inventory system for the Apollo missions. The DB I had to work with was created in the 80's. This project was in 2021, so it's still out there, supporting critical infrastructure.
IBM IMS isn't a network model database, it is a hierarchical model database.
The heyday of network model databases was after hierarchical, but before relational took over - basically the 1970s.
Hierarchical databases basically allow you to have parent-child relationships between tables, to represent many-to-one relations - something people commonly model nowadays using RDBMS foreign keys, but RDBMS foreign keys are much more flexible and can model other relationship types too, whereas hierarchical only allows parent-child.
Network model databases generalised hierarchical databases to allow more complex relationships than just parent-child. The biggest difference between them and the relational model, is they didn't have a query language like SQL, they offered a procedural API for processing one record at a time and manually navigating the links from table to table. Network model was standardised by CODASYL-the same committee which invented COBOL, with the result that they became particularly popular with COBOL applications.
The most famous network model database is probably Broadcom's IDMS (previously known as CA IDMS), which nowadays only runs on IBM and Fujitsu mainframes, but in past decades was supported on other mainframe and minicomputer platforms as well. Oracle also offers a CODASYL network model database for OpenVMS, which it got from Digital when it bought Rdb.
Graph databases have something in common with the historical network model, but some significant differences (1) graph databases have much more flexible schemas (defining arbitrary properties on a node as opposed to a rigid structure of fixed format record types), (2) graph databases generally have a query language with support for graph traversal operations, as opposed to the procedural API the network data model offered.
